Gas turbine engine starter generator that selectively changes the number of rotor poles

ABSTRACT

A rotating electrical machine, such as an aircraft starter-generator, that may be operated in either a motor mode or an generator mode. The machine includes a main rotor that is selectively configurable as an M-pole rotor or an N-pole rotor. The machine can also include DC brushes that are selectively moveable into, and out of, electrical contact the main rotor, to thereby electrically couple and decouple a DC power source to and from, respectively, the rotor windings.

FIELD OF THE INVENTION

[0001] The present invention relates to rotating electrical machinessuch as starter generators for gas turbine engines and, moreparticularly, to a starter generator that selectively changes the numberof rotor poles between operation as a DC motor and an AC generator.

BACKGROUND OF THE INVENTION

[0002] An aircraft may include various types of rotating electricalmachines such as, for example, generators, motors, and motor/generators.Motor/generators are used as starter-generators in some aircraft, sincethis type of rotating electrical machine may be operated in both a motormode and a generator mode. A starter-generator may be used to start theengines or auxiliary power unit (APU) of an aircraft when operating as amotor, and to supply electrical power to the aircraft power distributionsystem when operating as a generator. Thus, when operating as a motor, astarter-generator may be used to start the engines.

[0003] One particular type of aircraft starter-generator includes threeseparate brushless generators, namely, a permanent magnet generator(PMG), an exciter generator, and a main motor/generator. The PMGincludes permanent magnets on its rotor. When the PMG rotor rotates, ACcurrents are induced in stator windings of the PMG. These AC currentsare typically fed to a regulator or a control device, which in turnoutputs a DC current if the starter-generator is operating as agenerator. Conversely, if the starter-generator is operating as a motor,the control device supplies AC power.

[0004] If the starter-generator is operating in generator mode, DCcurrent from the regulator or control device is supplied to statorwindings of the exciter. As the exciter rotor rotates, three phases ofAC current are typically induced in the exciter rotor windings.Rectifier circuits that rotate with the exciter rotor rectify thisthree-phase AC current, and the resulting DC currents are provided tothe rotor windings of the main motor/generator. Finally, as the mainmotor/generator rotor rotates, three phases of AC current are typicallyinduced in the main motor/generator stator, and this three-phase ACoutput can then be provided to a load.

[0005] If the starter-generator is operating motor mode, AC power fromthe control device is supplied to the exciter stator. This AC powerinduces, via a transformer effect, an electromagnetic field in theexciter armature, whether the exciter rotor is stationary or rotating.The AC currents produced by this induced field are rectified by therectifier circuits and supplied to the main motor/generator rotor, whichproduces a DC field in the rotor. Variable frequency AC power issupplied from the control device to the main motor/generator stator.This AC power produces a rotating magnetic field in the main stator,which causes the main rotor to rotate and supply mechanical outputpower.

[0006] The above-described starter-generator may include relativelycomplex and heavy power electronics circuits in the control device. Forexample, some control devices may include inverters, for converting DCto AC power, rectifiers, for converting AC power to DC power, andpotentially complex voltage and frequency control circuits. Althoughbrush-type DC machines may alleviate the need for some of these complexand heavy electronic circuits, these also suffer certain drawbacks. Forexample, the brushes tend to wear fairly quickly, which can reducemachine reliability and increase the need for periodic maintenance andcleaning. Some brush-type DC machines can also suffer what is known astorque ripple during startup. In some instances, the torque ripple canbe large, which can result in poor starter performance.

[0007] Hence, there is a need for a starter-generator that does not relyon relatively complex and heavy inverters and frequency control circuitsfor proper operation, and/or does not suffer reduced reliability frombrush wear, and/or the need for potentially frequent maintenance andcleaning, and/or does not experience significant torque ripple duringstartup. The present invention addresses one or more of these needs.

SUMMARY OF THE INVENTION

[0008] The present invention provides a starter-generator that does notincorporate relatively complex power conversion and frequency controlcircuits, which reduces the weight and cost as compared to some currentstarter-generators. The starter-generator can increase the wear life ofits DC brushes, which reduces cleaning and maintenance, and thestarter-generator can provide a relatively smooth startup, whichimproves performance during startup.

[0009] In one embodiment, and by way of example only, a gas turbineengine starter-generator includes a housing, a main stator, and a mainrotor. The main stator is mounted within the housing. The main rotor ismounted within the housing, is located at least partially within atleast a portion of the main stator, and is selectively configurable aseither an M-pole rotor or an N-pole rotor.

[0010] In another exemplary embodiment, a gas turbine enginestarter-generator includes a housing, a shaft, a main rotor, a mainstator, a plurality of main rotor windings, and a switch. The shaft isrotationally mounted within the housing. The main rotor is mounted onthe shaft. The main stator is mounted within the housing and is locatedat least partially around at least a portion of the main rotor. Theplurality of main rotor windings are wound around at least a portion ofthe main rotor. The switch is electrically coupled between selected onesof the main rotor windings and has at least a first position and asecond position. When in the first position, the switch electricallycouples the main rotor windings such that the main rotor is configuredas an M-pole rotor and, when in the second position, electricallycouples the main rotor windings such that the main rotor is configuredas an N-pole rotor.

[0011] In yet another exemplary embodiment, a rotor for a gas turbineengine starter-generator includes a main rotor body and a plurality ofrotor coils that are wound around at least a portion of the main body.The rotor coils are wound in a configuration that allows the rotor to beselectively configured as an M-pole rotor or an N-pole rotor.

[0012] In still another exemplary embodiment, a motor/generatorincluding a stator having a plurality of stator windings wound around atleast a portion thereof and a rotor having a plurality of rotor windingswound around at least a portion thereof, is operated by a method thatincludes electrically coupling at least a portion of the rotor windingstogether such that the main rotor is configured as an N-pole rotor. DCpower is supplied to the electrically coupled rotor windings and to thestator windings, to thereby operate the motor/generator as a DC motor.At least a portion of the rotor windings are electrically coupledtogether such that the main rotor is configured as an M-pole rotor, andDC power is no longer supplied to the stator windings, to therebyoperate the motor/generator as an AC generator.

[0013] Other independent features and advantages of the preferredstarter-generator will become apparent from the following detaileddescription, taken in conjunction with the accompanying drawings whichillustrate, by way of example, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a simplified schematic block diagram of an exemplaryhigh speed starter-generator system according to an embodiment of theinvention;

[0015]FIG. 2 is a perspective view of a physical embodiment of thestarter-generator system depicted in FIG. 1;

[0016]FIG. 3 is simplified representation of a main rotor that may beused in the starter-generator of FIGS. 1 and 2, which schematicallydepicts various interconnections to the main rotor according to anexemplary embodiment of the present invention;

[0017]FIG. 4 is a simplified representation of the main rotor, similarto that shown in FIG. 3, with the rotor windings electrically connectedso that the main rotor is configured as an N-pole rotor during operationof the starter-generator system in a motor mode; and

[0018]FIG. 5 is a simplified representation of the main rotor, similarto that shown in FIG. 3, with the rotor windings electrically connectedso that the main rotor is configured as an M-pole rotor during operationof the starter-generator system in a generator mode.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0019] A functional schematic block diagram of one embodiment of amotor/generator system 100 is shown in FIG. 1. This exemplarymotor/generator system 100 includes an exciter 110, a mainmotor/generator 120, a motor/generator control unit 130, one or morerectifier assemblies 140, and one or more pairs of brushes 150. It isnoted that the motor/generator system 100 may be used as astarter-generator for a gas turbine engine in aircraft, space, marine,land, or other vehicle-related applications where gas turbine enginesare used. For aircraft applications, gas turbine engines are used forpropulsion (e.g., the aircraft's main engines) and/or for power (e.g.,the auxiliary power unit (APU)).

[0020] When the motor/generator system 100 is operating in generatormode, a rotor 122 of the main motor/generator 120, as will be describedmore fully below, is configured as an M-pole rotor, and the brushes 150are preferably moved out of physical contact with the mainmotor/generator rotor 122. The motor/generator control unit 130, whichis coupled to receive power from an input supply source 105, supplies DCpower to a stator 112 of the exciter 110, but is configured so that DCpower is not supplied to a stator 124 of the main motor/generator 120. Aprime mover 170 such as, for example, a gas turbine engine, rotates botha rotor 114 of the exciter 110 and the main motor/generator rotor 122.As the exciter rotor 114 rotates, it generates and supplies AC power tothe rectifier assemblies 140. The output from the rectifier assemblies140 is DC power and is supplied to a plurality of rotor windings 126wound on the main motor/generator rotor 122. As a result, AC power isoutput from stator windings 128 wound on the main motor/generator stator124.

[0021] During its operation in generator mode, the motor/generatorsystem 100 is capable of supplying output power at a variety offrequencies. Alternatively, a gearing system may be used to operate themotor/generator at a constant speed and, thus, supply a constantfrequency. The output power from the main motor/generator stator 124 istypically three-phase AC power. One or more stator output leads 125supplies the generated AC power to external systems and equipment viaone or more terminal assemblies 155. The motor/generator control unit130 can regulate the power output based upon monitoring signals providedto it from monitoring devices 195. In the depicted embodiment, theexciter 110 and the main motor/generator 120 both rotate along a singleaxis 198 at the same rotational speed. It will be appreciated, however,that in other embodiments the exciter 110 may rotate along a differentaxis. Moreover, the relative positioning of the exciter 110 and the mainmotor/generator 120 can be modified in different embodiments such thatthe exciter 110 is physically located on the other side of the mainmotor/generator 120.

[0022] When the motor/generator system 100 is operating in motor mode,the main motor/generator rotor 122 is configured as an N-pole rotor, andthe brushes 150 are moved into physical contact with the mainmotor/generator rotor 122. A DC power source 180, which is electricallycoupled to the brushes 150, supplies DC power to the mainmotor/generator rotor windings 126, via a commutator 129. The controlunit 130 is additionally configured to supply DC power to the mainmotor/generator stator windings 128, and no longer supply the DC powerto the exciter stator 112. It should be appreciated that the DC powerthat is supplied to the main motor/generator stator windings 128 maycome from the same, or a separate, DC power source that supplies thebrushes 150. In any case, as a result of this configuration, the mainmotor/generator rotor 122 is rotated, supplying rotational power to, forexample, the gas turbine engine 170. In the depicted embodiment, thebrushes 150 are moved in to, and out of, contact with the mainmotor/generator rotor 122 using an actuator 182, which is controlledusing, for example, brush control logic 184. In the depicted embodiment,the brush control logic 184 is located in the control unit 130, thoughit will be appreciated that it could be located elsewhere. It willadditionally be appreciated that the actuator may be any one of numerousknown types of actuators including, but not limited to, electrical,pneumatic, and hydraulic actuators. A perspective view of an exemplaryphysical embodiment of at least those portions of the motor/generatorsystem 100 that are mounted within a housing 200 is illustrated in FIG.2.

[0023] Turning now to FIG. 3, a simplified representation of the mainmotor/generator rotor 122, schematically depicting various switchedinterconnections between the plurality of main motor/generator rotorwindings 126, is shown. It will be appreciated that the rotor 122 andcommutator 129 are typically cylindrical in shape; however, for clarityand ease of explanation, each is shown in a flat, linear configuration.The commutator 129 includes a plurality of commutator segments 302 a-dthat are separated from one another by electrical insulators 304. A pairof the moveable brushes 150 is shown using both solid lines and dottedlines. The dotted lines are included to illustrate that the brushes 150,when in contact with the commutator 129, will contact each of thecommutator segments 302 a-d in turn, as the main motor/generator rotor122 rotates.

[0024] The main motor/generator rotor 122 includes a main body 306. Aplurality of pole segments 308 a-d, around which individual ones of therotor windings 126 a-d are respectively wound, extend from the rotormain body 306. The rotor windings 126 a-d are then electrically coupled,as described more fully below, to generate desired magnetic fieldpolarities in each of the pole segments 308 a-d when current flowsthrough the rotor windings 126 a-d. It is noted that, for clarity, onlya single turn of each of the individual rotor windings 126 a-d is shownon each pole segment 308 a-d. However, it will be appreciated that morethan one turn of rotor winding 126 a-d may be wound on each pole segment308 a-d.

[0025] In the depicted embodiment, the two rotor windings 126 a and 126d that are wound around pole segments 308 a and 308 d, respectively, areelectrically coupled in series between two commutator segments 302.Specifically, rotor winding 126 a is electrically coupled in seriesbetween commutator segments 302 a and 302 c, and rotor winding 126 d iselectrically coupled in series between commutator segments 302 b and 302d. The remaining two rotor windings, 126 b and 126 c, which are woundaround pole segments 308 b and 308 c, respectively, are eachelectrically coupled in series between a single commutator segment 302and a switch 310. Specifically, rotor winding 126 b is electricallycoupled in series between commutator segment 302 b and switch 310, androtor winding 126 c is electrically coupled in series between commutatorsegment 302 c and switch 310.

[0026] Switch 310 has at least two positions, a motor position (M) and agenerator position (G). In FIG. 3, however, the switch 310 is shown in atransition state between the two positions. In the motor position (M),switch 310 electrically couples rotor winding 126 b in series betweencommutator segments 302 b and 302 d, and rotor winding 126 c betweencommutator segments 302 a and 302 c. Conversely, in the generatorposition (G), switch 310 electrically couples rotor winding 126 b inseries between commutator segments 302 a and 302 b, and rotor winding126 c between commutator segment 302 c and 302 d. As FIG. 3 also shows,switch 310 also electrically couples the DC power source 180 in seriesbetween a pair of the moveable brushes 150 when in the motor position(M), and removes the DC power source 180 when in the generator position(G). Although shown as being separate wafers of the same switch, it willbe appreciated that switch 310 could be implemented as a plurality ofindividual switches. The switch 310 is remotely controlled by switchcontrol logic 186, which may be located in the control unit 130.However, it will be appreciated that the switch control logic 186 may belocated elsewhere. It should further be appreciated that the switch 310may be any one of numerous controllable switch types including, but notlimited to, a mechanical switch, a relay, and various types oftransistors. It should additionally be appreciated that the switch 310may be physically located within the controller 130 or external thereto,as shown in FIG. 3.

[0027] With the above-described electrical interconnection scheme, themain motor/generator rotor 122 may be selectively configured as either a4-pole rotor and a 2-pole rotor. The specific electricalinterconnections for these two different configurations will now bedescribed. Before doing so, however, it is to be appreciated that therotor structure and electrical interconnection scheme depicted anddescribed is merely exemplary of one that may be used to provide a4-pole/2-pole rotor combination, and that the rotor structure andelectrical interconnection scheme can be modified to provide any one ofnumerous M-pole/N-pole rotor combinations.

[0028] Referring first to FIG. 4, the configuration of the mainmotor/generator rotor 122 during operation of motor/generator system 100in motor mode is shown. In the motor mode, the switch 310 is in themotor position (M), and the main motor/generator rotor 122 is configuredas a 2-pole rotor. Specifically, as was noted above, with the switch 310in the motor position (M), rotor winding 126 b is electrically coupledin series between commutator segments 302 b and 302 d, and rotor winding126 c is electrically coupled in series between commutator segments 302a and 302 c. In the motor position (M), the DC power source 180 is alsoelectrically coupled in series between the pair of moveable brushes 150,which are moved into physical contact with the commutator 129. As wasalso noted above, when the motor/generator system 100 is operating inmotor mode, the control unit 130 is configured to no longer supply DCpower to the exciter stator 112. Thus, no power is supplied to thecommutator 129 via the rectifiers 140.

[0029] With the configuration shown in FIG. 4, current flows from the DCpower source 180, through one of the brushes 150 and commutator segment302 a, through rotor winding 126 a and rotor winding 126 c, back throughanother one of the brushes 150 and commutator segment 302 c, and back tothe DC power source 180. This flow of current generates a south magneticpole in pole segment 308 a and a north magnetic pole in pole segment 308c. It is noted that at the point in time depicted in FIG. 4, commutatorsegments 302 b and 302 d are not electrically coupled to receive powerfrom the DC power source 180, and no current flows through rotorwindings 126 b and 126 d. Thus, the main motor/generator rotor 122 isconfigured as a 2-pole rotor.

[0030] However, it should be appreciated that as the commutator 129rotates, the brushes 150 will move into contact with commutator segments302 b and 302 d, and out of contact with commutator segments 302 a and302 c. When this occurs, current flows from the DC power source 180,through commutator segment 302 b, through rotor winding 126 b and rotorwinding 126 d, through commutator segment 302 d, and back to the DCpower source 180. The main motor/generator rotor 122 is still configuredas a 2-pole rotor, but in this instance, the flow of current generates asouth magnetic pole in pole segment 308 b and a north magnetic pole inpole segment 308 d. Because commutator segments 302 a and 302 c, at thispoint in time, are not electrically coupled to receive power from the DCpower source 180, no current flows through rotor windings 126 a and 126c. This configuration is shown in phantom FIG. 4.

[0031] Turning now to FIG. 5, the configuration of the mainmotor/generator rotor 122 during operation of motor/generator system 100in generator mode is shown. In generator mode, the switch 310 is in thegenerator position (G), and the main motor/generator rotor 122 isconfigured as a 4-pole rotor. Specifically, as was noted above, with theswitch 310 in the generator position (G), rotor winding 126 b iselectrically coupled in series between commutator segments 302 a and 302b, and rotor winding 126 c is electrically coupled in series betweencommutator segment 302 c and 302 d. In the generator position (G), theDC power source 180 is also electrically decoupled from the pair ofmoveable brushes 150, which are moved out of physical contact with thecommutator 129. As was also noted above, when the motor/generator system100 is operating in generator mode, the control unit 130 is configuredto supply DC power to the exciter stator 112. Thus, DC power is suppliedto the commutator 129 via the rectifiers 140.

[0032] With the configuration shown in FIG. 5, current flows from therectifiers 140, through commutator segment 302 a, rotor winding 126 a,commutator segment 302 c, rotor winding 126 c, and through commutatorsegment 302 d back to the rectifiers 140. The current flow throughcommutator segment 302 a also flows through rotor winding 126 b,commutator segment 302 b, rotor winding 126 d, and through commutatorsegment 302 d back to the rectifiers 140. This flow of current generatessouth magnetic poles in pole segments 308 a and 308 c, and northmagnetic poles in pole segments 308 b and 308 d. Thus, the mainmotor/generator rotor 122 is configured as a 4-pole rotor.

[0033] Typically, when the motor/generator system 100 is beingimplemented as an aircraft starter-generator, the aircraft is on theground and the starter-generator is initially operated in a DC motormode. To do so, the switch 310 is moved to the motor position (M), thecontrol unit 130 is configured so that DC power is not supplied to theexciter stator 112, and the brushes 150 are moved into contact with themain rotor 122. Thus, the main rotor 122 is configured as a 2-polerotor, the DC power source 180 supplies DC excitation power to thestator windings 128, and to the rotor windings 126 via the brushes 150.The rectifiers 140 inhibit the DC power supplied to the brushes 150 fromreaching the exciter stator 112. The flux interaction between the rotorwindings 126 and the stator windings 128, and the commutation providedby the DC brushes 150 and commutator 129, gives rise to rotor 122rotation.

[0034] When the rotational speed of the rotor 122 reaches apredetermined magnitude and is increasing, the motor/generator system100 switches to operation in a generator mode. To do so, the switchcontrol logic 186 automatically moves the switch 310 to the generatorposition (G), and the control unit 130 is configured to supply DC powerto the exciter stator 112. In addition, the brush control logic 184causes the actuator 182 to move the brushes 150 out of contact with therotor 122. The AC power output from the exciter stator 112 is rectifiedby the rectifiers 140, and is supplied to the main rotor 122, which isconfigured as a 4-pole rotor.

[0035] It will be appreciated that the predetermined rotational speed atwhich operation switches from the motor mode to the generate mode mayvary, depending on the type of engine that is being started. It willadditionally be appreciated that this is only exemplary of a particularpreferred embodiment, and that the motor/generator 100 could also beswitched based on other operational needs, such as, for example, aspecified time period after it begins operating in motor mode.

[0036] While the invention has been described with reference to apreferred embodiment, it will be understood by those skilled in the artthat various changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt to a particularsituation or material to the teachings of the invention withoutdeparting from the essential scope thereof. Therefore, it is intendedthat the invention not be limited to the particular embodiment disclosedas the best mode contemplated for carrying out this invention, but thatthe invention will include all embodiments falling within the scope ofthe appended claims.

We claim:
 1. A gas turbine engine starter-generator, comprising: ahousing; a main stator mounted within the housing; and a main rotormounted within the housing and located at least partially within atleast a portion of the main stator, the main rotor selectivelyconfigurable as either an M-pole rotor or an N-pole rotor.
 2. Thestarter-generator of claim 1, further comprising: a control circuitelectrically coupled to at least the main rotor and operable toselectively configure the main rotor as the M-pole rotor or the N-polerotor.
 3. The starter-generator of claim 2, further comprising: aplurality of windings wound on at least a portion of the main rotor; anda switch electrically coupled between selected ones of the main rotorwindings, the switch having at least a first position and a secondposition, wherein the main stator is configured as the M-pole rotor withthe switch in the first position, and is configured as the N-pole statorwith the switch in the second position.
 4. The starter-generator ofclaim 3, wherein the control circuit comprises: switch control circuitryoperable to move the switch between at least the first position and thesecond position.
 5. The starter-generator of claim 5, furthercomprising: at least two brushes adapted to electrically couple to a DCpower source and selectively moveable into, and out of, electricalcontact with at least a portion of the main rotor, whereby the brushesare electrically coupled to, and decoupled from, respectively, the mainrotor windings.
 6. The starter-generator of claim 5, further comprising:an exciter rotor mounted on the shaft; an exciter stator having aplurality of windings wound thereon, the exciter stator mounted withinthe housing and located at least partially around at least a portion ofthe exciter rotor; and at least one rectifier assembly electricallycoupled in series between the exciter stator windings and the main rotorwindings.
 7. The starter-generator of claim 1, wherein M is unequal toN.
 8. The starter-generator of claim 7, wherein N=(M/2).
 9. A gasturbine engine starter-generator, comprising: a housing; a shaftrotationally mounted within the housing; a main rotor mounted on theshaft; a main stator mounted within the housing and located at leastpartially around at least a portion of the main rotor; a plurality ofmain rotor windings wound around at least a portion of the main rotor;and a switch electrically coupled between selected ones of the mainrotor windings and having at least a first position and a secondposition, wherein the switch, when in the first position, electricallycouples the main rotor windings such that the main rotor is configuredas an M-pole rotor and, when in the second position, electricallycouples the main rotor windings such that the main rotor is configuredas an N-pole rotor.
 10. The starter-generator of claim 9, furthercomprising: a control circuit operable to selectively move the switchbetween the first and second positions.
 11. The starter-generator ofclaim 10, wherein the control circuit includes: switch control circuitryoperable to selectively move the switch between the first and secondpositions.
 12. The starter-generator of claim 20, further comprising: aplurality of main stator windings wound on at least a portion of themain stator, each of the main stator windings adapted to selectivelyelectrically couple to a DC power source.
 13. The starter-generator ofclaim 9, further comprising: at least two brushes adapted toelectrically couple to a DC power source and selectively moveable into,and out of, electrical contact with at least a portion of the mainrotor, whereby the brushes are electrically coupled to, and decoupledfrom, respectively, the main rotor windings.
 14. The starter-generatorof claim 9, further comprising: an exciter rotor mounted on the shaft;an exciter stator having a plurality of windings wound thereon, theexciter stator mounted within the housing and located at least partiallyaround at least a portion of the exciter rotor; and at least onerectifier assembly electrically coupled in series between the exciterstator windings and the main rotor windings.
 15. The starter generatorof claim 9, further comprising: a DC power source electrically coupledin series with the main rotor when the main rotor is configured as anN-pole rotor.
 16. The starter-generator of claim 9, wherein M is unequalto N.
 17. The starter-generator of claim 16, wherein N=(M/2).
 18. Arotor for a gas turbine engine starter-generator, comprising: a mainrotor body; and a plurality of rotor coils wound around at least aportion of the main body, wherein the rotor coils are wound in aconfiguration that allows the rotor to be selectively configured as anM-pole rotor or an N-pole rotor.
 19. The rotor of claim 18, furthercomprising: a switch electrically coupled between selected ones of therotor coils and having at least a first position and a second position.20. The rotor of claim 19, wherein the rotor is configured as the M-polerotor with the switch in the first position, and is configured as theN-pole rotor with the switch in the second position.
 21. The rotor ofclaim 19, wherein M is unequal to N.
 22. The rotor of claim 21, whereinN=(M/2).
 23. A device for placement in a gas turbine enginestarter-generator, comprising: a controllable actuator moveable betweenat least a first position and a second position; and one or more DCbrushes coupled to the actuator.
 24. The device of claim 23, wherein:the actuator is adapted to receive a control signal; and the actuator isresponsive to the control signal to move between at least the first andsecond positions.
 25. In a motor/generator including a stator having aplurality of stator windings wound around at least a portion thereof anda rotor having a plurality of rotor windings wound around at least aportion thereof, a method of operating the motor/generator, comprising:electrically coupling at least a portion of the rotor windings togethersuch that the main rotor is configured as an N-pole rotor; supplying DCpower to the electrically coupled rotor windings and to the statorwindings, to thereby operate the motor/generator as a DC motor; andelectrically coupling at least a portion of the rotor windings togethersuch that the main rotor is configured as an M-pole rotor and no longersupplying DC power to the stator windings, to thereby operate themotor/generator as an AC generator.
 26. The method of claim 25, whereinthe motor/generator further includes at least two brushes that areselectively electrically coupled to, and decoupled from, the rotorwindings, the method further comprising: electrically coupling thebrushes to the rotor windings when operating the motor/generator as a DCmotor; and electrically decoupling the brushes from the from rotorwindings when operating the motor/generator as an AC generator.
 27. Themethod of claim 25, wherein the motor/generator changes from operationas a DC motor to an AC generator when its rotational speed reaches apredetermined magnitude.
 28. The method of claim 25, wherein themotor/generator changes from operation as a DC motor to an AC generatora predetermined time after commencing operation as a DC motor.